A 2D Large-Eddy Simulation Study of the Stratocumulus to Trade-Cumulus Transition

A two-dimensional Large-Eddy Simulation (LES) is used to investigate the dynamics of the transition from stratocumulus to trade-cumulus clouds in the subtropical oceans. Lagrangian simulations are performed by varying the sea surface and upper-air conditions following climatological or synoptic trajectories. In this way the evolution of a small slice of the cloud-topped boundary layer can be studied as it evolves along a trajectory. Here we consider boundary layer trajectories following the summertime subtropical highs in the FIRE IFO and ASTEX study regions.

The most influential climatological factor along these trajectories is the warming of the sea surface relative to the air above the inversion. This causes a deepening and subsequent decoupling of the stratocumulus boundary layer in the LES runs. After decoupling and the resultant surface layer moistening, cumulus clouds form and sometimes rise up into the overlying stratus deck. The vigorous circulations accompanying these convective bursts may play a central role in the eventual breakup of the stratocumulus deck. Diurnal variations in solar radiation are not required to produce the decoupling and cumulus formation observed in the LES. This is consistent with the predominance of decoupled boundary layers observed during the ASTEX experiment even at night. We compare the occurrence of decoupling in the LES model with mixed-layer model predictions. We also present preliminary simulations of the ASTEX Lagrangian experiments.